What is the best way to scale UVs to fit an image?
Grab to corner + Scale (not preserving proprtions)

With the Rotation/Scaling Pivot set to 2D Cursor and the cursor in the origin (0,0), activate the Constraint to Image Bounds option, grab the island in the corner and then scale:





Pack island+Scale (preserving proportions)

As stated by the Blender Reference Manual:


  The Pack Islands tool, shortcut Ctrl+P, will uniformly scale, then individually transform each Island so that they fill up the UV space as much as possible.




You can select multiple UV island to scale them while maintaining their relative proportions, or even just one as in the example to quickly scale every possible previusly unwrapped island to the bounds being sure it will not be stretched.

Notice that the box in the toolshelf is unchecked to prevent island's rotation.